Einstein Gravitational-wave Telescope

EINSTEIN TELESCOPE ON THE NATIONAL ROADMAP

FOR LARGE-SCALE RESEARCH FACILITIES

Conceptual diagram of the Einstein Telescope facility. Three detectors are configured in a triangular topology, and each detector consists of two interferometers. The 10 km arms of the observatory are housed underground to suppress seismic and gravity-gradient noise. Optical components are placed in an ultra-high vacuum and cryogenic environment. This proposal makes the case to realize Einstein Telescope in the Netherlands.

1/21

KNAW-Agenda Grootschalige Onderzoeksfaciliteiten Format nadere uitwerking van een ingezonden voorstel

I. GENERAL INFORMATION Acronym ET Name infrastructure Einstein Telescope Main applicant Prof.dr. Jo van den Brand Also contact person Organisation Nikhef and VU University Amsterdam Function Professor Address Science Park 105, 1098 GW Amsterdam, The Netherlands Phone +31 620 539 484 Email [email protected] Co applicants: Henk Jan Bulten, Nikhef and VU University Amsterdam Martin van Beuzekom, Nikhef Chris Van Den Broeck, Nikhef Niels van Bakel, Nikhef Alessandro Bertolini, Nikhef Jan Willem van Holten, Nikhef and Leiden University Paul Groot, Radboud University Gijs Nelemans, Radboud University

Summary

Einstein Telescope is a new infrastructure project that will bring Europe to the forefront of the most promising new development in our quest to fully understand the origin and evolution of the Universe, the emergence of the field of Gravitational Wave Astronomy. Gravitation is the least understood fundamental force of nature. Challenges include discovery of new sources and exploitation of gravitational waves, experimental constraints on the corresponding quantum (graviton) and the development of an observation-based field of research on quantum gravity. We propose that Einstein Telescope is realized in the Netherlands (part of Euregio Maas-Rhein) and will be an underground international facility containing cryogenic interferometers with 10 km arms.

We propose a phased approach where Phase I will allow qualification of sites in the Netherlands. After successful site selection, Phase II will involve construction, followed by exploitation in Phase III.

Keywords

Gravitational waves, fundamental physics, astronomy, astrophysics, black holes, early Universe, data analysis, laser interferometry, computing

2/21

II. PROPOSAL

A. SCIENCE AND TECHNICAL CASE Einstein Telescope in the Netherlands

Einstein Telescope (ET) is a new infrastructure project that will advance our understanding of the origin and evolution of the Universe by the detection and exploitation of gravitational waves. We propose that ET is realized in the Netherlands (as part of Euregio Maas-Rhein). It will be an advanced underground international facility containing cryogenic laser interferometers with 10 km arms.

The Einstein gravitational–wave Telescope will be an observatory of the third generation aiming to reach a sensitivity for gravitational wave signals emitted by astrophysical and cosmological sources about a factor of 10 better than the design sensitivity of the current second generation LIGO and Virgo advanced detectors, and will cover an extended frequency range from 2 to 104 Hz. Recently, the LIGO Virgo Collaboration has detected the first gravitational wave events from merging black holes. With its outstanding sensitivity, the ET observatory will open the era of routine gravitational wave astronomy with hundred thousands of detections per year.

ET will be realized in a phased approach. In Phase 0 a conceptual design study for ET was carried out in the FP7 framework call (see http://www.et-gw.eu/etdsdocument ). An R&D proposal ET was approved by ApPEC in 2012. Moreover, a Governing Council was instituted and a scientific collaboration was organized through the ET Science Team. Furthermore, ET was included on various roadmaps.

At present we are in Phase 1. An international community for ET exists with the required expertise to realize this facility. To organize and focus this community an integration proposal is being prepared that will be submitted to Horizon 2020 in 2016. In addition various countries have begun a systematic studies of sites to host the observatory.

The hosting of ET in the Netherlands (as part of Euregio Maas-Rhein) would have enormous benefits for the Dutch science community, society, and the Limburg region. Phase I of our proposal involves a detailed investigation of possible sites in the Netherlands to construct ET. Approval of Phase I would allow Dutch scientists to prepare the case for ET in the Netherlands. Phase 2 involves construction, while scientific exploitation of the observatory occurs in Phase 3.

For millennia the Universe has been studied with light and other forms of electromagnetic radiation. ET will open a new window on the Universe and since gravitational waves penetrate all regions of time and space with almost no attenuation, ET can sense waves from the densest regions of matter, the earliest stages of the Big Bang, and the most extreme warpings of spacetime near black holes.

Science Case

ET will feature advanced ground-based interferometers that will see gravitational wave observations firmly embedded in the wider field of astronomy and astrophysics. Enhancing detector performances beyond those achievable with current instruments will make it possible to continuously observe the distant, dark, dense and catastrophic Universe. Detectors capable of observing binary black hole (BBH) mergers will have an enormous impact in several key areas of astrophysics, cosmology and fundamental physics.

3/21

Targeted sources of gravitational waves: ET is primarily conceived to be a broadband detector, with good sensitivity over a significant part of the frequency range of 2–104 Hz. There are many classes of potential sources that are of great interest over this range. Binary neutron stars will sweep through the detector band from 2 Hz to 4 kHz, the signal lasting for several days as the system inspirals to a catastrophic merger event. The long observation times will make it possible to predict the location of the source and the precise time (to within milliseconds) when the system would coalesce, thereby facilitating simultaneous observation of the final merger of the two neutron stars using all windows of astronomical observation. Binary black holes will also last for several hours, up to a day, again making it possible to observe such events using optical, radio and other telescopes. Transient astronomical sources that are powerful emitters of high energy gamma-ray bursts and X-rays could also be visible in the gravitational window and reveal the inner structure of such sources. For instance, by observing the fluid modes of compact objects it would be possible to measure the equation of state of matter under extreme environments of gravity, magnetic field, temperature, etc. With a network of interferometers of capabilities similar to ET it should be possible to measure a stochastic gravitational wave background whose energy density is as small as 10–11 times that of the critical density of the Universe. ET will provide a new window on the Universe and can be expected to detect sources that have never been seen or imagined before.

Volume coverage and completeness of surveys: ET can basically be regarded as a survey telescope. ET will be able to constantly watch the entire gravitational sky, pointing of sources made possible either with the help of data from a network of three or more detectors, e.g. in the US and Australia, or by virtue of the modulation in the signal caused due to the detector’s motion relative to the source. Indeed, at its best conceived configuration ET will observe binary neutron star mergers within a red- shift of z = 2, binary black hole sources to a distance of z = 17, and all millisecond period neutron stars in the Galaxy with ellipticities larger than 10–8.

Strong field tests of Einstein’s gravity: Recent research has shown that some of the strong field tests of general relativity are possible only with the help of binary inspiral events that have signal-to-noise ratios (SNRs) in excess of about 100. Events with such high SNRs are not thought to occur frequently enough in current detectors. ET should be able to observe binary inspirals with SNRs of 100s about once each year. A single ‘gold-plated’ event will help explore the various nonlinearities of general relativity in ways that would never be possible with other terrestrial or solar-system experiments or radio binary-pulsar observations. Moreover, the higher harmonics that are present in the waveform will reveal the nature of the spacetime geometry in strong gravitational fields (as would result when compact stars merge) and help us formulate fundamental questions about the end-product of a gravitational collapse, Black hole mergers are the most powerful events in the Universe, and their if it is a black hole as gravitational waves allow stringent tests of general relativity.

4/21

predicted by general relativity or a more exotic object such as a naked singularity – a state in which a highly singular region of spacetime geometry is not shrouded in a horizon.

Astronomy: ET should observe a variety of different sources that should help resolve decades-old astronomy questions. The most important one amongst these is the origin of gamma-ray bursts (GRBs), pioneered by the Dutch astronomer Jan van Paradijs, which have remained an enigma nearly four decades after their serendipitous discovery in the 60’s and 70’s. Because of ET’s sensitivity to binary inspirals at high red-shifts, it should be possible to pin down the origin of GRBs and to confirm or rule out the association of binaries of NS-NS/BH and systematically study and understand different classes of GRBs. At the high frequency end of ET’s sensitivity, oscillations of neutron stars could be used to carry out asteroseismology and study the equation-of-state and the internal structure of matter at extreme environs of density, temperature and magnetic fields. Finally, the large amount of NS and BH mergers will provide accurate tests of the preceding binary evolution, which includes very uncertain phases. In turn, this will make it possible to understand the binary evolution of many other (high-energy) phenomena, such as X-ray binaries and type Ia supernovae.

Cosmology: ET will observe compact binary mergers over cosmological distances. As stated above, the distance reach for binary neutron stars is z = 2 and for binary black holes of total mass of 20 solar masses is z = 17. Observing a large number of sources at red-shifts of order 1 will help resolve the dark energy problem by measuring the expansion rate, or more precisely the acceleration rate, of the Universe at that time. This will be possible since compact binary sources are standard candles; the apparent luminosity, which depends on the distance to the source and the masses of the component stars, can be used to infer their luminosity distance, since chirping binaries allow a direct measurement of their masses. ET will also observe intermediate-mass black holes of total mass in the region of 1000 solar masses at a red-shift of z ~ 3. Therefore, ET can be used to ask whether the seed black holes at galactic nuclei were intermediate-mass black holes, what their mass function is and if supermassive black holes of billions of solar masses that are now found at galactic nuclei, are the result of merger histories and what those merger histories are.

Relation with other facilities and methods

Directly observing the minute spacetime ripples caused by a passing gravitational wave requires astonishing precision: typically 1 part in 1021. First attempts to directly detect gravitational waves started in the 1960s, using ton-scale bars designed to resonate at around a kHz after a gravitational wave passed thru. They failed. From the 1980s onwards, the focus has shifted towards long-baseline (3-5 km) laser interferometry with broadband sensitivity in the 100-1000 Hz region for observatories on Earth. In space the same technique can be used with sensitivities in the 10-4-1 Hz range.

First generation interferometers on the ground ran until 2010-11. No gravitational waves were detected, in accord with predicted gravitational-wave source populations  notably compact objects  within the sensitivity horizon of these instruments. Nevertheless, these projects established the infrastructures, the key technologies needed to attain the required precision and, equally important, forged a closely collaborating global community  overseen by the Gravitational Wave International Committee  ready to exploit the gravitational-waves physics discovery potential. In particular, LIGO and Virgo operations are now conducted through the LIGO Virgo Collaboration, with coordinated data- taking periods, data sharing, joint data-analyses, and co-authorship of publications.

Next generation ground-based interferometer projects, Advanced LIGO (USA), Advanced Virgo (Italy),

5/21

GEO-HF (Germany), and KAGRA (Japan), have been funded. Advanced LIGO already started its first science operations in 2015, while Advanced Virgo will follow in 2016. The sensitivity of these two instruments is expected to ramp up very rapidly to levels commensurate with multiple detections per year: gravitational-wave physics is about to take off! Detection rates and source localization will be enhanced if LIGO-India (foreseen for post-2020) becomes a reality and when KAGRA joins the already established LIGO-Virgo network.

Now that the first direct observation of gravitational waves is a fact, R&D is progressing towards full third- generation gravitational-wave observatories. On Earth, the Einstein gravitational wave Telescope, project planned for the 2020s, is the most advanced project. It will probe a thousand times larger volume than Advanced LIGO and Virgo and thereby elevate gravitational- wave physics from weekly or monthly detections to an era of high-statistics allowing high- precision astronomy and confronting the theory of General Relativity with a plethora of Strain noise estimates for the ground based detectors. The experimental measurements. 'LIGO-III' trace refers to an upgrade of Advanced LIGO.

The gravitational wave spectrum spans twenty decades in frequency: at the lowest frequencies, corresponding to the age of the Universe, the polarization of the cosmic microwave background should contain signals from the primordial gravitational waves due to cosmic inflation. The nano- to micro- Hertz band is covered by timing of pulsars and satellites. This method for detection is approaching maturity and is based on systematic monitoring of pulsars in so-called pulsar timing arrays. The Westerbork and LOFAR radio telescopes are part of the European Pulsar Timing Array, with involvement of ASTRON (Janssen, Hessels, Smit).

Between the timing measurements and the ground based detectors, the wide 10-5 to 1 Hz band will be pursued with space-based interferometers in the near future. Of all of the proposed sources of gravitational radiation, the most exciting one for cosmologists is perhaps the early Universe. For a scale invariant spectrum of radiation, we have the best chance of detection at low frequencies. Unfortunately, the astrophysical foreground of gravitational waves in the 10-9 to 10-1 Hz band makes the detection of an inflationary background nearly hopeless. Nearly all of the white dwarf binaries have merged before their orbital frequencies have increased to 0.1 Hz and so only the relatively small number of binaries containing neutron stars and black holes remain in the 0.1 to 1 Hz band. Two space missions are being studied to probe this frequency band: the Japanese Deci-Hertz Gravitational wave Observatory (DECIGO) and the international Big Bang Observer (BBO). In addition to the eventual detection of cosmological backgrounds, there is a wealth of astrophysical science which can be extracted during the foreground removal of these detectors. Unfortunately, it is unlikely that either of

6/21

these missions will fly within the next decade due to budgetary constraints.

In space, the undisputed flagship project is eLISA selected by ESA for its ‘Gravitational Universe’ mission with a launch date in 2034. eLISA is best adapted to gravitational waves in the 10-4-1 Hz range, thereby complementing the higher frequencies accessible to ground-based observatories. eLISA’s science program is very rich and includes the observation of coalescing supermassive black hole binaries out to redshifts of at least ten and the cannibalism of small black holes captured by supermassive black holes out to redshifts of about one. eLISA will also probe our own Milky Way galaxy, providing a census of the hundred million relativistic compact binaries (of white dwarfs, neutron stars, and stellar-mass black holes) estimated to exist in the Milky Way.

Comparison of strain noise estimates for future detectors: eLISA, DECIGO, BBO, Basic AGIS, and ET. The LIGO sensitivity curves are included for reference.

Europe has a leading role in ESA’s eLISA and LISA-Pathfinder space-based projects. ET can be considered the ground-based complement to the space-based eLISA mission. This is similar to the ground-based LSST facility complementing ESA’s Euclid satellite, and BICEP complementing the Planck satellite. eLISA is needed in order to detect individual sources with sizes larger than several km, such as supermassive black hole mergers, stars captured by supermassive black holes, and Galactic binaries. The RU Nijmegen group is the world-leading group for Galactic binaries and is part of the European eLISA consortium. A strategic partnership between SRON, Nikhef, NOVA, TNO, and various university groups has been set-up (the eLISA-NL consortium) to ensure a significant and focused Dutch contribution to this mission. A separate proposal for eLISA is submitted to this call.

An alternative to standard laser interferometry is to use clouds of atoms instead of mirrors. These clouds then take different free fall paths and the interference of the atomic clouds is used to read out

7/21

any gravitational wave signal. The advantages of these atomic techniques are many, but Newtonian noise is a problem for the atom interferometers just as it is for laser interferometers. A spaced based detector, the Atomic Gravitational wave Interferometric Sensor (AGIS), has been proposed to circumvent these terrestrial limits. It remains to be seen if this type of atom interferometry can be made to be competitive with other technologies (such as DECIGO).

The above discussion shows that realization of ET will be the final step of a long path and the initial act of a new scientific adventure for ground-based interferometers. Most importantly in order to proceed is that the advanced detectors confirm the effectiveness of their new technologies and that these interferometers detect gravitational waves.

The detection of gravitational waves has always been regarded as a prerequisite for the start of ET. Now this is accomplished and as a next step ET needs to be included on the international roadmaps (e.g. ApPEC and ESFRI). This can be expected in 2018 at the earliest. Then site selection must take place. For this reason the excavation of the ET site cannot start before 2022, and hence 2028 is here taken as the initial time for the ET observatory realization. ET being an observatory that will be on line for decades, priority in construction will be attributed to the site and infrastructures realization, selecting a modular philosophy for the detectors that will allow to implement the different interferometers composing each detector with a schedule stretched in time. In this way, after about 6 years of construction, installation and commissioning, the first detector of the ET observatory could be operational at the end of the next decade.

Roadmap showing the evolution of gravitational waves detectors in the World. Advanced LIGO features second generation interferometers and started operation in September 2015, together with GEO-HF. Advanced Virgo will join in 2016. KAGRA is under construction in Japan and will join the network in 2018. MIGA is based on atom interferometry, while TOBA represent torsion bars. ET could be operational as early as 2028. Cosmic Explorer is a third generation interferometer now under study in the USA. eLISA is a space-based interferometer that is expected to run in parallel with ET and Cosmic Explorer.

The design of the new detector must evolve from the current conceptual phase to the technical design phase, and various R&D activities must confirm the feasibility of the solutions proposed in the Conceptual Design Report. This includes instrumentation topics such as Newtonian noise subtraction techniques to improve the important low frequency region, development of silicon test masses and coatings, and new laser technologies.

8/21

Technical case

An artist’s impression of ET is given in the figure on the front page. The main purpose of the ET project is the realization of an infrastructure (an “observatory”) capable of hosting more than one gravitational wave detector. This infrastructure will be usable for many decades, while the implemented detectors will undergo successive upgrades or replacements depending on future developments of optical, and interferometer technologies.

To reduce the effect of the residual seismic motion, thus allowing a better sensitivity at low frequencies (between 2 and 100 Hz), ET will be located underground at a depth of about 100 to 200 m and, in the complete configuration, it will consist of three nested detectors, each in turn composed of two interferometers (a so-called xylophone configuration).

Optical layout of ET showing three detectors in a triangular topology. Each detector consists of two interferometers.

Each interferometer will have a dual-recycled Michelson layout with Fabry–Perot arm cavities, with a length of about 10 km. The xylophone configuration of each detector devotes one interferometer to the detection of the low-frequency components of the gravitational wave signal (2–40 Hz) while the other is dedicated to the high-frequency components, each interferometer adopting different, optimal

9/21

technologies. In the former (ET-LF), operating at cryogenic temperature, the thermal, seismic, gravity gradient and radiation pressure noise sources will be particularly suppressed; in the latter (ET-HF) the sensitivity at high frequencies will be improved through the high laser light power circulating in the Fabry–Perot cavities, and through the use of frequency-dependent squeezed light technologies. The sensitivity of the ET observatory, at the current level of understanding, will be as good as 3 x 10-25 /Hz at frequencies around 100 Hz.

One of Nikhef's responsibilities within the ET project has been to define the site and infrastructure. The observatory design needs to incorporate complex underground facilities, featuring multi-kilometer tunnels to host the interferometer arms and cryogenic plants.

The main target of the conceptual design has been the demonstration of the feasibility of the ET project, through the definition of the requirements and main characteristics of the hosting site, the design of the key components of the research infrastructure, the indication of the possible technologies and the presentation of the main detector design elements. Also a rough cost estimate was then reported to evaluate the financial feasibility of the ET project. For a detailed discussion of the technical aspects, we refer to http://www.et-gw.eu/etdsdocument .

Challenges and risks

R&D: The conceptual design phase was completed in 2011. However, a series of difficult steps must be expected. The first step is the consolidation of the design solving the several questions currently present in many technologies needed for ET. In fact the detectors in the ET observatory will adopt technologies that aren't explored in the advanced detectors, like cryogenics, Silicon mirrors, different wavelength lasers, optical squeezing, and gravity gradient noise subtraction techniques. The ET design consolidation needs, then, a technical design phase, supported by an intense R&D activity. This activity is ongoing and has been funded by ApPEC in 2012.

10/21

Funding: The ET community must find the (human and financial) resources to support these activities; these resources should be a mix of national and international funds (i.e. like the ApPEC R&D funds). In parallel to the progress of the various technologies, the conceptual design needs to evolve in a technical design, describing in detail the components of the observatory. This phase of the project will need a correct framework and, because of the fact that the national funding agencies in Europe are fully engaged with the realisation of the advanced detectors, a possible choice is a networking or integration tool at the European level (probably in Horizon 2020) in combination with an ERIC.

Roadmaps: To support a future for the ET project, it is strongly recommended to insert the project in the list of the major research infrastructures (the so-called ESFRI roadmap) recommended by the “European Strategy Forum on Research Infrastructures" (ESFRI). Already part of the activities, necessary to achieve this target, has been performed inserting ET in the specialised roadmaps, like the GWIC roadmap https://gwic.ligo.org/roadmap/) and the ApPEC roadmap http://www.aspera- eu.org/images/stories/files/Roadmap.pdf, but intense outreaching and proposition actions are still needed.

11/21

B. EMBEDDING Relation to Dutch research groups

Gravitational wave physics and ET have been part of Nikhef’s Strategic Plan for 2011 to 2016. Nikhef has been active in ET as a founding member and has been responsible for site studies from the beginning. Moreover, the Netherlands has made significant investments in Virgo during the last five years, with Nikhef, VU and RU-IMAPP taking important tasks and responsibilities both in instrumentation and in the development of analysis algorithms and pipelines for finding and studying gravitational wave signals from coalescing binaries. Nikhef has developed detailed studies of seismic and gravity gradient effects, and is at present the main institute that studies seismic noise suppression schemes for both Advanced Virgo and ET.

The emerging Dutch astroparticle physics community has recognized that a country like the Netherlands can only have a significant impact in the field if the research effort is focused on a limited number of projects. For that reason the “Strategic Plan for Astroparticle Physics in the Netherlands” was defined (updated in 2014; gravitational waves features prominently). From the beginning, gravitational wave physics was foreseen as one of the three pillars of the national program, together with (radio) detection of cosmic rays and deep-sea neutrino detection.

The involvement in these areas allows for a multi-messenger approach to astroparticle physics and should result in a strong and internationally visible research program at this new frontier in science in the Netherlands.

The gravitational wave activities of the Dutch astronomy, physics, and theory communities provide excellent opportunities for an interdisciplinary program between particle physics, cosmology and astronomy. Nikhef scientists focus on gravitational wave research for fundamental physics. We have organized annual meetings to foster coherence in the gravitational wave community. The first such national meeting took place at Nikhef on 19 January 2012, the second on 31 January 2013 at Radboud University in Nijmegen, the third on 7 February 2014 at ASTRON in Dwingeloo, the fourth on 20 February 2015 in Leiden, and the fifth occured in March 2016 at Nikhef.

Dutch research on particle physics and astronomy is of the highest scientific quality. Below we briefly mention Dutch research activities in gravitational waves beyond ET and LVC:

 The BlackGEM telescope array will consist of 20 optical telescopes dedicated to the follow-up of gravitational wave events. The Netherlands Research School for Astronomy (NOVA) has funded the design and production of phase-1 (3 telescopes) of the array.  Within NOVA Network 3, several gravitational wave related topics are studied, including astrophysical exploitation of BlackGEM. In addition, astrophysical sources of gravitational waves (compact binary systems involving neutron stars, white dwarfs and/or stellar mass black holes, as well as supermassive black holes in galactic nuclei) are studied in Amsterdam, Leiden and Nijmegen by using radio, near-IR, optical, UV, X-ray and gamma ray instruments.  The telescopes of Westerbork are part of the European Pulsar Timing Array, a collaboration that has recently been formed between the five major radio observatories in Europe: Jodrell Bank, Effelsberg, Westerbork, Nancay and Sardinia. The goal is detecting gravitational waves in the nano-Hertz regime by using high precision timing of an array of millisecond radio pulsars distributed across the sky. This allows studies of wide supermassive black hole binaries and stochastic signals of cosmic strings.  Many of the questions addressed by the theoretical astrophysics and astroparticle physics community in the Netherlands are closely linked to the gravitational physics science program.

12/21

The investigations range from the physics of black holes to astroparticle physics and physics beyond the Standard Model. We mention for example studies of the gravitational radiation emitted by supermassive black holes, cosmic accelerators, mechanisms for lepto- or baryogenesis, string theory, inflation and extra dimensions, and topological defects like cosmic strings.  Within SRON, the Netherlands Institute for Space Research, control and readout electronics for inertial sensors have been developed for the Inertial Sensor Test Module (ISTM) project of the LISA Pathfinder mission. SRON has also contributed to detailed software models of the Inertial Sensor, which are included in the industrial End-to-End Simulator developed at EADS Astrium (Germany). TNO has developed technologies that are crucial for the alignment and pointing of the optical path of the LISA Pathfinder.

The Netherlands is involved in eLISA with TNO developing systems that ensure the laser beams of eLISA to arrive at the exact location, even over a 5 million km distance. Nikhef will be involved in the phase meter and collaborates with TNO, NOVA, Twente University and SRON in technology development for eLISA. Moreover, scientists from Nikhef, Radboud University, University of Amsterdam, Leiden, VU University, Utrecht University, ASTRON and SRON join forces.

External access to the infrastructure, and data access

Detailed governance of the observatory and the roles and responsibilities of scientists maybe modeled on current LIGO and Virgo practice. This implies that institutions requesting to join the ET Scientific Collaboration should contact the Spokesperson. Prospective members must arrange an MOU with the ET Scientific Collaboration and present their proposed collaborative program at an ET meeting. New memberships may then be approved by the ET Council.

We anticipate that given the substantial public funds that must be invested in generating the data, data taken with the instruments of the ET observatory will be made available and useable to the broader research community.

No definite plans exist at this moment but there is a general recognition that the collected data constitute a rich treasure that must be protected, preserved, and exploited. Experience from other fields such as earth observations, climate modeling, and seismology, shows that society has profited from common data formats and open data. This will allow cross disciplinary research and will enable data mining. NASA has a well-established open data policy, while CERN is paving the road for a common infrastructure to allow data and resource sharing on a global scale via the Grid. Nikhef is a strong partner in this endeavor and operates a Tier-1 Grid computing facility.

13/21

The importance for society and/or industry and the connection with societal developments ET addresses some of the most fundamental questions asked by society, concerning the origin, composition and future of our Universe. This proposal seeks approval to investigate the possibility of realizing this observatory in the Netherlands (as part of Euregio Maas-Rhein). A world class large scale infrastructure such as ET would act as a magnet for the brightest minds and would clearly be an asset for an economically healthy country such as the Netherlands. Moreover there are specific benefits for the region Limburg in attracting significant number of scientists, engineers, technicians and their families (both in education, housing, and services like hotels/restaurants/airports). Moreover, such a large project would allow industry to profit. ET will bring together many disciplines in its quest for a deeper understanding of nature. Without doubt this will bring significant spin-off to society.

High precision interferometry has many applications. The state-of-the-art technologies for ET will be developed in close collaboration with industry. ET will house the largest ultra-high vacuum chamber in Europe (Virgo at present) with vessels tens of kilometers long. The construction requires the development of special metallurgical production processes and the realization of large welded tube assemblies with a high accuracy. Extremely low diffusion optical coatings on large substrates will be employed for the mirrors and gratings of ET.

We believe that South Limburg is the most suitable site location for ET. There the older rock (about 360 million years) is coming to the surface allowing the infrastructure to be realized in a cost effective manner in hard rock, while the local geology is well suited to guarantee the requirements for low seismic and gravity gradient noise for such a facility. Preliminary seismic studies have been carried out at the Heijmansgroeve, a site operated by KNMI.

Nikhef scientists in collaboration with KNMI, Bjorn Vink (local geoscientist), and Innoseis (a Nikhef start-up specialized in seismic sensors) carry out seismic studies at Heijmansgroeve, a site operated by KNMI in Limburg. Expertise in underground construction is vital for the project. At present we work closely with Grontmij (see http://www.grontmij.nl/Projecten/Pages/HuisadviseurvoorprojectA2Maastricht.aspx , responsible for the A2project), earth scientist Bjorn Vink (an expert on the local geology), and KNMI for seismic measurements.

We are in close contact with local leadership of Provincie Limburg and scientists from the Albert Einstein Institute in Hannover. There is a strong high tech industry “belt” of companies ranging from the Eindhoven area through Limburg, Aken, and central Germany with the required expertise in (underground) construction. The area has several universities (Maastricht, Aken, Luik) and the Einstein Telescope infrastructure may have access points in The Netherlands, Germany and perhaps Belgium. In addition, the area has excellent connections via Maastricht Airport as well as through highways and high-speed trains.

14/21

Seismic sensors have been developed for studies of seismic and gravity gradient noise and these sensors have found their way to the oil and gas industry for seismic exploration (e.g. Shell). Data analysis at ET draws heavily on both signal enhancing techniques and large scale computing (the excellent Dutch BIGGrid and e-Science infrastructure is a valuable asset). These techniques play an important role in digital image processing (the field of Computer Vision), with applications extending to the medical field. The cutting edge scientific and technological enterprise at ET is extremely suited to be positioned in outreach projects.

The importance for the Netherlands Discoveries with the highest scientific merit are expected with ET and this attracts the brightest minds in the world, from experimental and theoretical physics, astronomy, mathematics and computation, and other fields in science. It is clear that an advanced research facility as ET will be the center piece of these activities and the relevant scientific community will gravitate towards this site. ET will play a role for the Netherlands much like for example CERN for Switzerland, ESA, ESRF and ITER for France, and ESO, EMBL and XFEL for Germany.

In parallel to Nikhef’s effort, research groups in Hungary and Italy are pursuing site studies for ET. Recently a parallel effort for developing a third generation observatory has started in the USA. However we believe that a site in the Netherlands has various significant advantages. After the current facilities have made first direct gravitational wave detections, ET will feature prominently on the various European roadmaps (a new roadmap for Astro-particle physics is being prepared by ApPEC). Dutch scientists have the ambition to host this new facility, and we believe Phase I of this project should start as soon as possible. This will allow detailed studies of the underground seismic and geological properties, as well as technical and (underground) construction studies that will allow realistic budgetary estimates. Furthermore, it allows detailed studies of the economic advantages of such an observatory for the Netherlands.

Dutch research on particle physics and astronomy is of the highest scientific quality. The Nikhef group (Van Bakel, Van Beuzekom, Van den Brand, Bertolini, Bulten, Van Den Broeck) joined the Virgo collaboration in 2007 and became a member of the EGO Council in 2009. Its contributions to Virgo include angular alignment of optical components, optical systems for the input mode cleaner, seismic isolation systems, cryolinks, and phase cameras. Virgo reached its design sensitivity, with the ability to measure relative length changes of Δ L/L < 10–22. Right now the instrumental activities are focused on a major upgrade to the second-generation Advanced Virgo, which is poised to make first detections as part of a global network of detectors in 2016.

Nikhef is also strongly involved in the development of data analysis techniques. It is a dominant player in two out of four of Virgo’s analysis groups. The RU Nijmegen group (Nelemans, Groot) recently also became a full member of the Virgo collaboration and is working on the astrophysical interpretation of upcoming detection events.

The activities are embedded in the traditionally strong fundamental physics and astrophysics community in the Netherlands. The connection between gravitational waves, inflation, string theory, and cosmic defects is studied by Postma (Nikhef); new techniques for the calculation of waveforms are pursued by van Holten (Nikhef and Leiden); gravitational wave emission by neutron stars is studied by Watts (UvA); and Nelemans (RU Nijmegen) works on progenitors of compact binaries and the complementarity of electromagnetic observations.

15/21

C. ORGANISATION and FINANCES

Organisation

Founding institutions for ET are the main European stakeholders in gravitational waves research at both LIGO, GEO-HF, and Virgo:

European Gravitational Observatory Istituto Nazionale di Fisica Nucleare Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V., acting through Max- Planck- Institut für Gravitationsphysik Centre National de la Recherche Scientifique University of Birmingham University of Glasgow Nikhef Cardiff University

Representatives of these institutions are members of the Governing Council.

In parallel an ET Science Team was formed to investigate in detail the scientific potential of the observatory. At present more than 220 scientists from 57 institutions are listed as members of the ET Science Team.

International interested participants The search for gravitational waves is underway at Virgo (a French-Italian-Dutch detector) and GEO-HF (close to Hannover operated as a German-UK collaboration) in Europe, and at LIGO in the USA. The scientific endeavor is organized in a united approach through the LIGO Virgo Collaboration (LVC). The current LVC encompasses more than 1400 scientists from 130 institutions and 18 countries.

Institutions, and countries currently participating in gravitational wave physics with ground-based interferometers that might make use of the infrastructure, encompass current users of the LIGO and Virgo facilities.

We believe that the current LVC members represent a lower bound on the potential users of the infrastructure. Once the gravitational wave window on the Universe has been opened, and scientists can make a new class of observations by studying the tiny vibrations in the fabric of spacetime, this will attract significant numbers of scientists to a world class observatory such as ET.

In addition we have various international collaborations outside Europe. For example the ELiTES programme with Japanese institutes: http://www.et-gw.eu/descriptionelites

Furthermore, there are regular discussions with institutes in the USA organized through LIGO and LSC. Common meetings are organized on a regular basis (e.g. February 2 and 3, 2016 in Florence, and May 23 to 27 in Elba, Italy, and the Dawn Workshop on July 7 and 8, 2016 in Atlanta, U.S.A. The goal of these meetings is to discuss the scienctific potential of a worldwide network of third generation ground- based detectors. Representatives from US, Europe, Japan, India and Australia have been present.

16/21

Governance

The European Gravitational Observatory (EGO) hosts the Virgo interferometer. ET has up to now been governed by EGO, which coordinated the efforts between the institutions and the EU. During the Conceptual Design Study EGO instituted the Governing Council consisting of representatives from the founding institutions. We foresee that EGO will play a coordinating role, until an ERIC can be instituted for ET.

We propose the governance of ET to be modeled after successful examples in particle physics (e.g. CERN) and astronomy. Note that LIGO and Virgo have implemented a detailed governance for both the infrastructure and the scientific collaboration. For details see http://www.ligo.org/about/governance.php

17/21

Global cost estimate and financing

Scientists in The Netherlands, in collaboration with those in Italy, France, Germany, the United Kingdom, Switzerland and Spain, have begun study of the technologies for a 3rd generation observatory. Currently the best estimate for the cost of construction of this facility is M€ 1075.

At this stage of the conceptual design the costs of the Observatory have to be regarded as a rough estimate. A summary of the estimated costs is shown in the table. More details on the costing are explained in section 6.2 of the Conceptual Design Study for ET (see http://www.et- gw.eu/etdsdocument ).

The overall costs of an underground facility like the ET Observatory are dominated by excavation costs and construction of the underground tunnels and caverns. These costs depend significantly on the location and type of soil. In the design study prepared within FP7 a rather conservative assumption of € 260/m3 has been made for the excavation cost. The costs listed in the table assume a single detector to be implemented first. The costs include spares for each individual item. In most instances the spares will remain unused and can act as spares for the subsequent installation of the other two detectors, reducing their price tags somewhat.

Annual exploitation costs are estimated at about €30 million and have been scaled from current operating costs of LIGO and Virgo. This facility has been identified as a priority in the ApPEC Roadmap. ApPEC advises to the European funding agencies involved in astro-particle physics and maybe a natural vehicle to initiate discussions on cost-sharing issues.

18/21

D. FUTURE DEVELOPMENTS Main issues

ET’s conceptual design needs to evolve in a technical design, describing in detail the components of the observatory. This phase of the project will need a correct framework and, because of the fact that the national funding agencies in Europe are fully engaged with the realisation of the advanced detectors, a possible choice is a networking or integration tool at European level (for this we a Horizon 2020 proposal will be submitted in 2016) in combination with an ERIC. Recently, ET has been inserted in the specialised roadmaps, like the GWIC roadmap https://gwic.ligo.org/roadmap/) and the ApPEC roadmap http://www.appec.org . As a next step, it is strongly recommended to insert the ET project in both the various national roadmaps and in the list of the major research infrastructures (the so-called ESFRI roadmap) recommended by the “European Strategy Forum on Research Infrastructures".

The ET community must find the (human and financial) resources to support an intense R&D activity to explore various technologies such as cryogenic silicon mirrors, different wavelength lasers, optical squeezing, and gravity gradient noise subtraction techniques. The required resources should be a mix of national and international funds (i.e. like the ApPEC R&D funds).

Timeline

The ET design consolidation needs a technical design phase, supported by an intense R&D activity. This activity is ongoing and has been funded by ApPEC.

Expected timeline for ET. The Conceptual Design Report has been completed, while ongoing R&D has been funded via ApPEC. Advanced detectors have reached a sensitivity that will allow detection of gravitational waves events within the next years. This makes the discussion about (inter)national roadmaps timely.

An a priori condition for the funding of the construction phase is the detection of a gravitational wave signal in the advanced detectors. The 2015 - 2017 period was taken as a realistic time window for the detection of the first gravitational wave signal from a binary black hole (and/or neutron star) system by current facilities. This will be followed by a site preparation phase, where all the legal and preliminary aspects of the land ownership acquirement are exploited. In parallel the production and

19/21

test of the first detector hardware components will start in the laboratories participating in the ET project. The site and infrastructure realisation will last for about four years, followed by the first ET detector installation.

To save time, it is expected that part of the first detector installation (i.e. vacuum pipes deployment) could overlap with the latest infrastructure realisation activities. Thanks to the experience acquired with the initial GW detectors (that will be improved with the next commissioning of the advanced interferometers) it is possible to state that the commissioning phase will last at least for more than 3-4 years, with some early science data taking interlaced with the commissioning periods.

The observatory infrastructure and detectors will be designed to have in a modular approach for the components; this will allow sequential installation phases, interlaced with periods of data taking for the detectors, already operative in the ET site. In this way it will be also possible to upgrade the installed detectors when technological progress will make it convenient, maximising the duty cycle. This possibility underlines our main approach: to provide a Research Infrastructure operative for decades and that will be able to host evolving detectors.

20/21

ACRONYMS

AdvLIGO, AdvVirgo Second generation gravitational wave detectors in USA and Italy AGIS Space based Atomic Gravitational wave Interferometric Sensor ApPEC Astroparticle Physics European Consortium ASPERA Predecessor of ApPEC ASTRON Netherlands Institute for Radio Astronomy Astrium Aerospace manufacturer subsidiary of EADS BBH Binary black hole BBO Big Bang Observer (successor of eLISA) BICEP A large angular scale CMB polarimeter at the South Pole BlackGEM Optical telescopes under construction at RU-IMAPP Cosmic Explorer Third generation gravitational wave detector under study in US CDR Conceptual Design Report of ET CERN European Organization for Nuclear Research DECIGO DECI-Hertz Interferometer Gravitational wave Observatory under study in Japan EADS European Aeronautic Defence and Space Company (now Airbus Group SE) EGO European Gravitational Observatory (host of Virgo; Cascina, Italy) eLISA Evolved Laser Interferometer Space Antenna, previously called LISA EMBL European Molecular Biology Laboratory (Heidelberg, Germany) ERIC European Research Infrastructure Consortium (legal framework) ESA European Space Agency ESFRI European Strategy Forum on Research Infrastructures ESRF European Synchrotron Radiation Facility (Grenoble, France) ESO European Organisation for Astronomical Research in the Southern Hemisphere ET Einstein Telescope (consists of low- and high-frequency interferometers) Euclid ESA mission to map the geometry of the dark Universe FP7 Seventh Framework Programme for Research and Technological Development GEO-HF Upgrade of the GEO600 is a gravitational wave detector (Hannover, Germany) GRB Gamma Ray Burst Grid Form of distributed computing Grontmij Third largest engineering consultancy company in Europe (now part of Sweco) GWIC Gravitational Wave International Committee ISTM Inertial Sensor Test Module is part of eLISA ITER International Thermonuclear Experimental Reactor (Cadarache, France) KAGRA Kamioka Gravitational Wave Detector, formerly LCGT (Kamioka, Japan) KNMI Royal Netherlands Meteorological Institute L3 ESA Large Class mission (Gravitational Universe was selected as third mission) LHC Large Hadron Collider at CERN LIGO Laser Interferometer Gravitational-Wave Observatory (Hanford, Livingston, US) LIGO-III Possible future upgrade of LIGO LISA Predecessor of eLISA LOFAR Low-Frequency Array for Radio astronomy (built by ASTRON) LVC LIGO Virgo Collaboration MIGA Matter-wave laser Interferometric Gravitation Antenna MOU Memorandum Of Understanding NASA National Aeronautics and Space Administration (USA) NOVA Netherlands Research School for Astronomy RU-IMAPP Radboud University Institute for Mathematics, Astrophysics and Particle Physics SNR Signal to Noise Ratio SRON Netherlands Institute for Space Research Tier-1 Level of the Worldwide LHC Computing Grid TOBA Torsion-bar antenna for detecting low-frequency gravitational waves

21/21